The substitution of metal sites in Mg 2 TiO 4 substrate leads to charge imbalance that will be closely related to a variety of changes including lattice structure, cell distortion, and photophysical properties. Herein, the co-substitution strategy of [Ga 3+ −Ga 3+ ] for [Mg 2+ −Ti 4+ ] and Sn 4+ for Ti 4+ achieves for the first time the novel Mg 3 Ga 2 SnO 8 (MGS):xMn 4+ (x = 0−3%) phosphors with efficient red emissions. In terms of X-ray powder diffraction (XRD) and Rietveld refinement analysis, MGS:Mn 4+ possesses a structure isotypic of Mg 2 TiO 4 in the cubic space group Fd3̅ m (227). There are two types of octahedra for Mn 4+ ions in this structure, where Ga 3+ ions completely occupy a group of octahedral sites and Mg 2+ /Sn 4+ has been randomly distributed over another group of octahedral sites. A strong excitation band in the broad spectral range (220− 550 nm) has been identified, thus facilitating the commercial uses for blue LED chips excitation. An intense red emission band at 680 nm has been observed due to the characteristic 2 E g − 4 A 2g transition of Mn 4+ ions. A concentration quenching effect occurs when the Mn 4+ content exceeds 1.5%, and the quenching mechanism is demonstrated to be dipole−quadrupole interactions. Temperature-dependent luminescence measurements support its good thermal stability, and the corresponding activation energy E a is determined to be 0.2552 eV. The possible luminous mechanism of the Mn 4+ ion is explained by the Tanabe−Sugano energy level diagram. The crystal field strength and the Racah parameters together with the nephelauxetic ratio are also determined for Mn 4+ in the MGS lattice. High color rendition warm white-lightemitting diodes (WLEDs) based on the optimal phosphor MGS:1.5%Mn 4+ ,1.5%Li + possess a color rendering index and color temperature of 85.6 and 3658 K, respectively. Its feasibility for application in solid-state white lighting has been verified.
Metal-free catalyzed intermolecular tandem Michael addition/cyclization is developed for the synthesis of benzo[4,5]imidazo[1,2-a]pyridines from α-bromocinnamaldehyde. The RIM of the benzene ring at the C1-position makes them a new class of AIE molecules.
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